1. Field of the Invention
This invention relates to microencapsulated materials. More particularly, this invention relates to an improved method for preparing microencapsulated compounds of metals from the platinum group of the periodic table. Many of these encapsulated compounds are useful as hydrosilation catalysts for one-part, heat curable organosiloxane compositions.
2. Description of the Prior Art
One of the most useful classes of polyorganosiloxane compositions cure by a platinum catalyzed hydrosilation reaction. The advantages of these compositions relative to moisture curable polyorganosiloxane compositions is their rapid curing rate, particularly at temperatures above about 50 degrees C., and their ability to cure in thick sections. Compositions that cure by a hydrosilation reaction typically contain a polydiorganosiloxane with at least two ethylenically unsaturated hydrocarbon radical per molecule, an organohydrogensiloxane containing at least two silicon-bonded hydrogen atoms per molecule in an amount sufficient to achieve curing of the composition and a platinum-containing catalyst in an amount sufficient to promote curing of the composition. Fillers and other additives may be present for the purpose of modifying physical and/or chemical properties of the composition either prior to or following curing.
Because the aforementioned platinum catalyzed organosiloxane compositions begin to cure even at ambient temperature once the reactants are combined, the platinum catalyst and the organohydrogensiloxane reactant usually are packaged in separate containers and are not combined until it is desired to cure the composition. Even if the composition contains one or more of the known platinum catalyst inhibitors it cannot be stored in a single container for more than a few hours.
One of the alternatives proposed in the prior art to supplying platinum-catalyzed curable organosiloxane compositions as two-package materials is to isolate either the catalyst or the organohydrogensiloxane within a matrix of a material that is solid under the conditions encountered during storage of the curable composition, and which allows the entrapped reactant or catalyst to escape and mix with the other ingredients when it is desired to cure the composition.
U.S. Pat. No. 4,481,341, which issued to Schlak et al. on Nov. 6, 1984 describes thermosetting organosiloxane compositions comprising a polyorganosiloxane containing at least two ethylenically unsaturated hydrocarbon radicals per molecule, a polyorganohydrogensiloxane containing at least two silicon bonded hydrogen atoms per molecule and a platinum-containing catalyst that is dispersed in a finely divided, solid material, such as a silicone resin, at a concentration of from 0.001 to 5 percent by weight of platinum metal.
The finely divided material in which the catalyst is dispersed is virtually insoluble in either the aforementioned polyorganosiloxane or polyorganohydrogensiloxane and melts or softens at a temperature between 70 and 250 degrees C. The alleged advantage of these compositions disclosed by Schlack et al. is that the catalyst remains isolated from the other ingredients of the curable composition until the composition is heated sufficiently to melt the material in which the catalyst is dispersed. Because the organosilicon compounds present in the composition will not cure in the absence of the catalyst, the composition can allegedly be stored for long periods of time without undergoing curing or even an increase in viscosity.
A disadvantage of the curable organosiloxane compositions described by Schlak et al. is the presence of an incompatible resin in the curable composition. The maximum platinum content disclosed for the catalyst/resin mixture is 5 percent by weight, and is 0.1 percent in the single exemplified composition. The catalyst mixture is prepared by combining 100 parts by weight of a solubilized silicone resin with 10 parts of a solution of the platinum catalyst in isopropanol. The solution contains 1 percent by weight of the catalyst, based on the platinum metal content. This requires a relatively large amount of resinous carrier material in addition to the catalyst. By definition the carrier is insoluble in the other organosilicon compounds present in the composition. The carrier may therefore detract from the appearance and/or properties of the cured material.
A second potential disadvantage of Schlack et al. catalyst resides in the method used to prepare the catalyst composition. A solid block or sheet of resin containing the platinum composition dispersed throughout is ground to a fine powder. The random nature of the grinding operation makes it possible that some of the particles will contain platinum catalyst on their surface. Even trace amounts of platinum have been shown to cause premature curing of the type of organosiloxane composition exemplified in this patent.
One way to avoid the inherent disadvantages of the catalyst compositions described in the Schlack et al. patent, is to completely microencapsulate finely divided particles or droplets of a catalyst composition within a material that is impermeable to the catalyst and effectively isolates it from the reactive ingredients of a curable organosiloxane composition. The encapsulant melts or softens at the desired curing temperature of the composition.
The prior art describes several methods for microencapsulating materials, including organosilicon compounds, utilizing either chemical or mechanical means. Chemical means typically include precipitation, polymerization and/or curing of the encapsulant in the presence of the material to be encapsulated. The material to be encapsulated is present as a dispersed phase of particles or droplets in a liquid medium and the processing conditions are such that the encapsulant forms a continuous, solid film around each suspended particle or droplet.
Useful encapsulants include thermoplastic and thermosetting organic polymers, crosslinkable gelatinous and resinous materials such as gelatin and organic waxes that melt or soften at temperatures above about 50 degrees C.
The prior art discloses one-part curable organosiloxane compositions containing microencapsulated reactants or catalysts. One example of this type of art is U.S. Pat. No. 4,528,354, which issued to McDougal and Dougherty on July 9, 1985. This patent teaches a method for curing one-part peroxide curable silicone rubber compositions. The compositions include a microencapsulated liquid phase containing an organic peroxide in a shell of a resinous thermosetting material that is impervious to the peroxide. The capsules are designed to rupture under a given internal vapor pressure that is generated by the encapsulated liquid when the curable composition containing the microcapsules is heated.
Because release of the peroxide is dependent upon rupturing rather than melting of the shell separating the peroxide from the other ingredients of the organosiloxane composition, the composition and thickness of the shell must be carefully controlled to ensure that rupture of the capsules will occur reproducibly within the temperature range used to cure the organosiloxane composition.
European Published patent application No. 140,770 which issued on May 8, 1985 describes storage stable polyorganosiloxane compositions comprising a polyhydroxylated polysiloxane, a polyacyloxysilane and a microencapsulated accelerator. The coating material of the microcapsules can only be penetrated using heat and/or irradiation. Suitable encapsulating materials include polystyrene, acrylonitrile-styrene copolymers, and polymethyl methacrylate. This publication does not suggest using microencapsulated materials in organosiloxane compositions curable by means other than the reaction of polyhydroxylated polyorganosiloxanes with acyloxysilanes.
U.S. Pat. No. 4,293,677, which issued to Imai on Oct. 6, 1981 describes encapsulating organohydrogensiloxanes using complex coacervation and in-situ polymerization, two of the most common microencapsulation techniques. In accordance with Example 1 of this patent, an aqueous gelatin solution adjusted to a pH of 9.5 was added to a solution of a trimethylsiloxy terminated polymethylhydrogensiloxane in molten paraffin wax. A 10 percent by weight solution of gum arabic is then added and the pH of the resultant emulsion is adjusted to 4 over a period of two hours to precipitate the mixture of the two polyelectrolytes that forms the encapsulant. The encapsulant is cured by gradually heating the dispersion of coated particles to a temperature of 50 degrees C.
The in-situ polymerization process exemplified in Example 2 of the Imai patent involves the polymerization of styrene in the presence of a dimethylsiloxane/methylhydrogensiloxane copolymer as the dispersed phase of an emulsion wherein the aqueous phase contains a solubilized polyvinyl alcohol and potassium persulfate.
One of the most effective classes of platinum-containing catalysts for curing organosiloxane compositions of the type described in the Imai et al. patent are reaction products of an inorganic platinum compound such as hexachloroplatinic acid with liquid vinyl-containing organosilicon compounds such as sym-tetramethyldivinyldisiloxane. The solution is then diluted to the desired platinum content, typically between 0.1 and 1 percent by weight, using a liquid dimethylvinylsiloxy terminated polydimethylsiloxane.
When the present inventors attempted to microencapsulate one of these preferred platinum catalyst compositions by combining the composition with a solution of polystyrene in a water immiscible liquid, emulsifying the resultant composition in water and then evaporating the water immiscible liquid to precipitate the polystyrene as a coating around the droplets of catalyst composition, the resultant microcapsules were agglomerated and could not be isolated as a free flowing powder.
Washing the microcapsules with organic solvents such as toluene and methanol did little to reduce the extent of agglomeration. Curable compositions containing these microcapsules, a vinyl-containing liquid polydimethylsiloxane and a dimethylsiloxane/methylhydrogensiloxane copolymer exhibited very poor storage stability under ambient conditions, indicating that some of catalyst was either initially present on the surface of the microcapsules or had diffused through the encapsulant during storage of the curable composition.
The present inventors continued their investigations with the objective of preparing microencapsulated platinum-containing compounds that could be isolated as a free flowing powder with substantially no platinum present on the surface of the microcapsules, and which could be used to prepare storage stable organosiloxane compositions curable by a hydrosilation reaction.